chore: update openvm build nightly version

[jonathanpwang]

For guest programs with MSRV 1.87+, it is convenient for cargo openvm build to use a newer Rust toolchain. We chose 2025-08-02 since that is when the last stable Rust 1.90.0 branched from master: https://releases.rs/docs/1.90.0/

Note that users can always override the toolchain version with env var OPENVM_RUST_TOOLCHAIN.

workflow run: https://github.com/axiom-crypto/openvm-reth-benchmark/actions/runs/18085897389/job/51456939479

[Copilot]

Pull Request Overview

Updates the default nightly Rust toolchain version used by cargo openvm build from nightly-2025-02-14 to nightly-2025-08-01 to better support guest programs with MSRV 1.87+.

• Update default Rust nightly toolchain to nightly-2025-08-01
• Document the toolchain change in the changelog

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/toolchain/build/src/lib.rs	Updates the DEFAULT_RUSTUP_TOOLCHAIN_NAME constant to use the newer nightly version
CHANGELOG.md	Adds entry documenting the toolchain version update

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[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(-32 [-12.5%]) 225	322,610	2,058,654	-	-	-
fibonacci	(+37 [+3.8%]) 1,023	1,500,209	(-7560 [-0.4%]) 2,100,402	-	-	-
regex	(+13 [+0.6%]) 2,352	(+28022 [+0.7%]) 4,137,372	(+40192 [+0.2%]) 17,759,394	-	-	-
ecrecover	(-6 [-0.9%]) 697	(-14273 [-10.4%]) 122,859	(-14284 [-0.6%]) 2,263,820	-	-	-
pairing	(+38 [+2.7%]) 1,438	(-137197 [-7.3%]) 1,745,742	(-376960 [-1.5%]) 25,468,210	-	-	-

Commit: 434ad0d

Benchmark Workflow